A polyester (hereinafter sometimes abbreviated as PET resin) having a main repeat unit of ethylene terephthalate is used as a material for containers of, for example, carbonated drink, juice, mineral water and the like, due to the superior properties it has in transparency, mechanical strength, heat resistance, gas barrier property and the like, and has been dominantly used for these purposes. For this use, a polyester bottle is hot-filled with beverage sterilized at a high temperature, or the beverage is sterilized at a high temperature after filling. Typical polyester bottles suffer from shrinkage and deformation during hot filling treatment and the like. The heat resistance of polyester bottles may be improved by a proposed method comprising imparting higher crystallinity by a heat treatment of the bottle finish part, or heat-fixing the bottles after stretch blowing. These methods, nevertheless, are associated with the possibility of leakage of the content as a result of poor sealability of the cap, particularly when finish part shows insufficient crystallization or when the crystallization degree varies greatly.
For an improved heat resistance of the bottlebody, moreover, a heat treatment is applied by raising the temperature of stretch blow mold, as shown in JP-B-59-6216. When a number of bottles are formed using the same mold according to such method, however, the obtained bottles increasingly whiten with longer hours of operation. This has a consequence that the transparency is degraded to finally produce bottles without a product value. It has been elucidated that a cyclic trimer derived from a PET resin attaches to the surface of the mold and stains the mold, and this mold stain transfers to the surface of the bottles. Particularly, since a higher forming speed is employed for the production of smaller bottles in recent years, the mold stain is posing a growing problem from the aspect of productivity.
In addition, when the content requires hot filling, such as in the case of fruit juice, the finish part of a preform or molded bottle is generally heat treated for crystallization (method described in JP-A-55-79237 and JP-A-58-110221). Such method for increasing the heat resistance by heat treating the finish part and the shoulder part is subject to variation in productivity depending on the time and temperature of the crystallization treatment. Thus, the PET resin preferably has a high crystallization rate to allow treatment at a low temperature in a short time. The bottlebody is required to be transparent even after heat treatment during forming, so that the tone of the packed product will not be degraded. Therefore, the finish part and the body need to have opposite properties.
Various propositions have been made to solve these problems. For example, a method comprising addition of an inorganic nucleating agent such as kaolin and talc to polyethylene terephthalate (JP-A-56-2342, JP-A-56-21832), a method comprising addition of an organic nucleating agent such as montanic wax and the like (JP-A-57-125246, JP-A-57-207639) and the like have been proposed, but these methods accompany foreign substance and clouding, posing problems in actual application. In addition, a method comprising inserting a heat resistant resin piece into the finish part (JP-A-61-259946, JP-A-2-269638) has been proposed, but the bottle shows poor productivity and there are problems of recyclability.
The problems of staining of a mold has been conventionally dealt with by reducing cyclic trimer, which is the main component of the substance attached to the mold surface, by subjecting PET resin to solid phase polymerization in advance. This method, nevertheless, offers only an insufficient effect, as cyclic trimer is regenerated by re-melting during parison molding. In U.S. Pat. Nos. 5,219,984, 5,241,046 and 5,270,444, a method for suppressing the activity of a catalyst and the generation of cyclic trimer during parison molding is disclosed, which comprises treating a polyester resin with water at 90-110.degree. C. However, this method requires a special apparatus and time for the treatment, making the production process complicated. While the above-mentioned method reduces stains of the mold to some extent, the effect is not sufficient and, in some cases, the effect was unnecessary.
During the water treatment, the fine particles (resin fine powder) attached to the polyester chip are suspended or precipitated in water during the treatment, which then attach to the wall of treatment tanks and pipes, thus clogging the pipes and making cleaning of the treatment tanks and pipes difficult.
The fine particles suspended or precipitated in water treatment and then attached to the wall of treatment tanks and pipes attach to the polyester chips again. This has a consequence that crystallization is promoted during forming and the transparency of the bottles becomes poor. In addition, crystallization of a finish part results in the shrinkage of the finish part outside the standard level, causing failure of capping.
When this method is industrially used, distilled water is economically disadvantageous as the water for treatment, and industrial water prepared by simplified treatment of water from river, underground water, gray water and the like is generally used. When industrial water is used for the water treatment, however, crystallization during forming occurs too early only to produce bottles having poor transparency. In addition, crystallization of finish part may result in the shrinkage of the finish part to the level outside the standard range, causing failure of capping.
According to the study of the present inventors, when the content of a metal-containing substance [e.g., sodium (Na), calcium (Ca), magnesium (Mg), silicon (Si) and the like] in the industrial water is above a certain level during water treatment, a metal-containing substance (e.g., oxide and hydroxide), which is called a scale of these metals, suspends or precipitates in the treatment water and attaches to the wall of the treatment tank and pipe. The attached substance adheres to and penetrates into the polyester resin chips, promoting crystallization during forming, and making the transparency of the bottles poor. Further problems included clogging of a pipe with the metal-containing substance, and difficult cleaning of the treatment tanks and pipes. When Na is contained, scales are not produced but Na ion penetrates into the surface layer of the chip and crystallization is accelerated about the Na ion as mucleus, causing whitening of the bottle. The content of these metal-containing substances increases after rain, varies depending on the season and sometimes rises very high. The content also varies greatly depending on the source of industrial water.
Even for a resin for bottles wherein the catalyst has not been deactivated by water treatment, water having high hardness is sometimes used when the strand is chipped, but the transparency does not decrease to a significant level. However, the low transparency due to the above-mentioned foreign substance is particularly noticeable in the bottle wherein the catalyst has been deactivated by water treatment. The catalyst reacts with water and is deactivated thereby, as a result of which the catalyst converts to particles insoluble in resin. The particles become crystal nuclei that brings about a synergistic effect by the promotion of crystallization.